The present invention relates to plasma discharge etching equipment of the type which is often used to process semiconductor wafers.
Plasma discharge devices have been employed for some time to etch wafers for integrated circuits and other semiconductor devices. This plasma etching apparatus has a chamber which is evacuated and into which an ionizable gas then is introduced. A pair of opposing electrodes within the chamber are excited by a radio frequency signal to produce a glow discharge plasma between the electrodes. The plasma of gas ions is created by ionizing electrons colliding with the gas atoms or molecules within the chamber. The ionizing electrons are produced by the impingement of gas ions on the semiconductor wafer and its supporting electrode causing a secondary emission of electrons from those objects. These secondary electrons also collide with gas atoms or molecules in the chamber generating additional ions and a sustained plasma.
The charged ions are accelerated toward one of the electrodes on which the semiconductor wafer to be etched is placed. A combination of the force of the ions striking the wafer and chemical processes erodes the wafer material. A mask may be deposited on the surface of the wafer so that the erosion is confined to areas which are not covered by the mask.
The ionizable gas within the etching apparatus typically is maintained at a relatively high pressure in order to maximize the probability that ionizing electrons will strike a gas atom or molecule. Previous devices maintained a very close separation between the two electrodes to increase the plasma density for a fixed input power and gas pressure. However, a minimum electrode separation has to be observed so that the separation is greater than the ionization mean free path of the electrons. Therefore, a trade off in the separation distance between the two electrodes exists whereby a close separation is desirable from a plasma confinement standpoint, while a larger separation is required in order to increase the probability that an electron will travel a sufficient distance to collide with a gas atom or molecule. Low pressure is also desirable because it results in better directionality of etching as a result of the absence of energetic ion collisions. Furthermore it is desirable to have as large and uniform a plasma as possible in order to etch large wafers and produce more semiconductor devices per etching operation.